There are known in the prior art instruments for measuring the moisture content of a moving web in an on-line installation. In such instruments a microwave transmitting horn on one side of the web directs microwave energy at a frequency of about 22.2 GHz through the web to a receiving horn on the opposite side of the web. The signal output of the receiving horn is used to provide an indication of the moisture content in the web. One example of such a microwave moisture measuring instrument is disclosed in Mounce U.S. Pat. No. 3,851,244, issued Nov. 26, 1974. Other moisture measuring instruments are shown in Walker U.S. Pat. No. 3,693,079, Busker et al U.S. Pat. No. 3,681,684 and Walker U.S. Pat. No. 3,534,260. In these latter patents various devices are provided to prevent reflected radiation from returning to the transmitting horn effectively to augment or decrease the apparent power radiated from the sending horn.
In systems of the type discussed hereinabove, power P at the receiving horn is related to power Po from the sending horn by the relationship: EQU P/Po=e.sup.-.alpha.t (1)
where .alpha. is the attenuation constant and t is the thickness of the attenuating medium. The attenuation constant of the web is given by: ##EQU1##
From the foregoing it will be seen that microwave moisture sensors of the prior art depend for their operation on the interaction of microwaves with the water in the web.
While microwave moisture measuring instruments of the prior art are generally satisfactory in providing an indication of the moisture content of a moving web, they suffer from a number of defects. They do not account for the effects on the radiation of properties of the web other than moisture content.
Microwaves couple to rotational degrees of freedom of water molecules which are hindered by interactions with the matrix or base material of the web and with other water molecules. For example, when the web is a cellulose product, water molecules bind to hydroxyl groups on the fiber. The bound water interacts differently with the radiation than the unbound water. Since the relative amount of bound to unbound water is determined by the equilibrium temperature of the web, the attenuation of radiation by the cellulose-water mixture is temperature dependent.
In addition to the interaction of the microwaves with water in the web, the radiation also interacts with the base matrix of the web so that the attenuation constant depends on the dry weight or dry mass per unit area of the web.
When the thickness of the web is near a multiple of one fourth the wavelength of the microwaves in the medium, reflection from the surface of the web results in a dependence of the attenuation constant on web thickness.
None of the moisture measuring systems of the prior art take into account the effects of web thickness on the attenuation constant with the result that the indications provided are not as accurate as is desired.